Photosensitive resin compositions have been used for forming protective films to prevent electronic parts such as TFT type liquid crystal display elements, magnetic head elements, integrated circuit elements, solid state image sensor, and the like from being degraded or damaged and for forming interlayer dielectric films to insulate a space between layered wires. Among these uses, in particular, liquid crystal display elements are produced by a method comprising the steps of forming a transparent conductive circuit layer of ITO or the like on an interlayer dielectric film and further forming a liquid crystal alignment film on the same. The interlayer dielectric film is subjected to high temperature conditions during the step of forming a transparent electrode film and is exposed to a resist stripper liquid used in electrode pattern formation which it must be sufficiently resistant to. Consequently, it is required that photosensitive resin compositions used for forming interlayer dielectric films must have excellent developability and can be formed into films with excellent transparency, heat resistance, planarization properties, adhesion properties, chemical resistance and electrical characteristics.
In addition, microlenses having a lens diameter of about 3 to 100 μm or microlens arrays comprising regularly aligned microlenses are used as imaging optics of on-chip color filters of facsimile apparatuses, electronic copying machines, solid state image sensor, and the like or optical materials for optical fiber connectors. Among them, CCD elements used in digital cameras and the like have been further miniaturized recently as the number of pixels has been increased and the light-receiving area per CCD element has been reduced. Therefore, the formation of convex shape of microlenses on CCD elements has been studied as a means for increasing the quantity of received light. A method for forming microlenses used in CCD elements comprising exposing a photoresist film to light, developing the exposed film to form concave and convex patterns, heating the concave and convex patterns to a temperature higher than the glass-transition point to fluidize the same, and forming a hemispherical microlens by surface tension is known. Therefore, it is required that photosensitive resin compositions used in forming microlenses should have excellent developability and should also have transparency, melt flow properties, heat resistance as well as a high refractive index in order to impart light collecting properties.
Regarding liquid crystal displays, recently, there has been increased use of semi-transmissive and reflective liquid crystal displays, compared with conventional liquid crystal displays with backlight. A reflective plate such as an aluminum plate or the like is provided under a liquid crystal panel and light received from the upper portion of the panel is reflected onto the reflective plate to provide brightness. However, if the reflective plate is flat meaning that light obliquely incident on the panel surface is reflected in an oblique direction, and thus a sufficient axial brightness cannot be obtained. In order to solve this problem, a method comprising forming concave and convex patterns using a photosensitive resin as a base material for a reflective plate and forming a metal film on the same to prepare a light diffusive and reflective film has been examined. A high level of axial brightness can be achieved by this method, when obliquely incident light is used. Therefore, photosensitive resins used for forming light diffusive and reflective films should have concave and convex patterns, the heat resistance required for performing metal sputtering, solvent resistance, and the like.
Further, TN-type TFT-LCDs have been the most widely used liquid crystal displays. Levels of axial contrast and color reproducibility are the same or higher than those of cathode-ray tubes. However, they have the defect of a narrow viewing angle. In order to solve this problem, vertically aligned mode liquid crystal displays have been developed. Slits are provided in pixel areas of a display side electrode and pyramid-shape alignment control protrusions made of a photosensitive resin are formed on a light incident side electrode. As a liquid crystal adjacent to the alignment film can be kept standing vertically by this method, the viewing angle becomes wider and contrast is improved. In addition, a rubbing treatment step to align the liquid crystal is not necessary. Photosensitive resins used for forming alignment control protrusions should have an appropriate cross-sectional shape and are required to have a high degree of properties such as resistance to solvents including a solvent used in the subsequent alignment film formation step, heat resistance to heat applied during the step of forming alignment films, transparency, resolution, residual film ratios, and the like.
Further, bead-shaped spacers are used in liquid crystal displays in order to keep the distance between two transparent electrode substrates constant. The spacers are dispersed on either of the two transparent electrode substrates when they are bonded to each other and the cell gap changes depending on the diameter of the spacers. In order to avoid problems such as non-uniform cell gap and the like caused by a significantly varying diameters of the spacers, methods for forming spacers using photosensitive resins have been studied. It is required that the photosensitive resins used for forming spacers should have a high degree of properties such as transparency, resolution, residual film ratio, and the like as well as appropriate cross-sectional shapes.
There is a technical means using unsaturated carboxylic acids such as methacrylic acid and the like as components imparting alkali solubility in the above photosensitive resin compositions (Patent publications 1 to 5). However, it is difficult to manage the developability of these photosensitive resin compositions and there are also problems with their storage stability.
Patent publication 6 describes photosensitive resin compositions comprising copolymer (a) of hydroxystyrene and methyl methacrylate, thermally curable resin (b) reactive with copolymer (a), and photosensitizing agent (c). However, the photosensitive resin compositions described in the publication have a defect in that they are colored upon heating.
The inventors of the present application developed photosensitive resin compositions with excellent transparency, heat resistance, and the like in order to solve the technical problems regarding photosensitive resin compositions described in these patent publications (Patent publication 7).